HELP PLEASE I BEG YOU 100 POINTS

Which is the completely factored from of xy^3-x^3y

How to use order of operation in 20 divide by 4 multiply 5 = 1

Astudent must have an average on five test that is greater than it equal to 80% but less than 90% to receive a final grade of b. devon's greades on the first four test were 78% 62% 91% and 80% what range if grades on the fifth test would give him a b in the course? ( assuming the highest grade is 100%)